Chronic exposure to inhaled anesthetics increases cholesterol content in Acholeplasma laidlawii

Molecular mechanisms of drug action: an emerging view

Volatile anesthetics serve as useful probes of a conserved biological process that is essential to the proper functioning of the central nervous system. A kinetic and thermodynamic analysis of their unusual pharmacological and physiological characteristics has led to a general, predictive theory in which small molecules that adsorb to membranes modulate ion channel function by altering physical properties of membrane bilayers. A kinetic model that is both parsimonious and falsifiable has been developed to test this mechanism. This theory leads to predictions about the structure, function, origin, and evolution of synapses, the etiology of several diseases and disease symptoms affecting the brain, and the mechanism of action of several drugs that are used therapeutically. Neuronal membranes may offer an appealing drug target, given the large number of compounds that adsorb to interfaces and hence membranes.

Animal models and conserved processes

BACKGROUND

The concept of conserved processes presents unique opportunities for using nonhuman animal models in biomedical research. However, the concept must be examined in the context that humans and nonhuman animals are evolved, complex, adaptive systems. Given that nonhuman animals are examples of living systems that are differently complex from humans, what does the existence of a conserved gene or process imply for inter-species extrapolation?

METHODS

We surveyed the literature including philosophy of science, biological complexity, conserved processes, evolutionary biology, comparative medicine, anti-neoplastic agents, inhalational anesthetics, and drug development journals in order to determine the value of nonhuman animal models when studying conserved processes.

RESULTS

Evolution through natural selection has employed components and processes both to produce the same outcomes among species but also to generate different functions and traits. Many genes and processes are conserved, but new combinations of these processes or different regulation of the genes involved in these processes have resulted in unique organisms. Further, there is a hierarchy of organization in complex living systems. At some levels, the components are simple systems that can be analyzed by mathematics or the physical sciences, while at other levels the system cannot be fully analyzed by reducing it to a physical system. The study of complex living systems must alternate between focusing on the parts and examining the intact whole organism while taking into account the connections between the two. Systems biology aims for this holism. We examined the actions of inhalational anesthetic agents and anti-neoplastic agents in order to address what the characteristics of complex living systems imply for inter-species extrapolation of traits and responses related to conserved processes.

CONCLUSION

We conclude that even the presence of conserved processes is insufficient for inter-species extrapolation when the trait or response being studied is located at higher levels of organization, is in a different module, or is influenced by other modules. However, when the examination of the conserved process occurs at the same level of organization or in the same module, and hence is subject to study solely by reductionism, then extrapolation is possible.

Tolerance to isoflurane does not occur in developing Xenopus laevis tadpoles

INTRODUCTION

Tolerance is observed for a variety of central nervous system depressants including ethanol, which is an anesthetic, but has not been convincingly demonstrated for a potent halogenated volatile anesthetic. Failure to demonstrate tolerance to these agents may be the result of inadequate exposure to anesthetic. In this study, we exposed Xenopus laevis tadpoles to surgical anesthetic concentrations of isoflurane for 1 wk.

METHODS

Xenopus laevis tadpoles were produced by in vitro fertilization, and exposed to isoflurane (0.59%, 0.98%, 1.52%) or oxygen for 1 wk starting from the time of fertilization.

RESULTS

Changes in anesthetic EC(50) were small and not in a consistent direction. Control animals had an anesthetic EC(50) of 0.594% +/- 0.003% isoflurane. Tadpoles exposed to 1.52% isoflurane had a lower EC(50) than controls (by 16%), whereas tadpoles raised under 0.59% and 0.98% isoflurane had higher EC(50)s than control (by 4.7% and 7.4%, respectively).

CONCLUSION

We provide the first description of week-long exposures of vertebrates to surgical anesthetic concentrations of isoflurane, and the first report of such exposures in developing vertebrates. Tolerance to isoflurane does not occur in developing Xenopus laevis tadpoles. Taken together with studies in other organisms, the development of tolerance to ethanol but not isoflurane suggests that mechanisms shared by these drugs probably do not account for the development of tolerance.

A hypothesis on the origin and evolution of the response to inhaled anesthetics

In this article, I present an evolutionary explanation for why organisms respond to inhaled anesthetics. It is conjectured that organisms today respond to inhaled anesthetics owing to the sensitivity of ion channels to inhaled anesthetics, which in turn has arisen by common descent from ancestral, anesthetic-sensitive ion channels in one-celled organisms (i.e., that the response to anesthetics did not arise as an adaptation of the nervous system, but rather of ion channels that preceded the origin of multicellularity). This sensitivity may have been refined by continuing selection at synapses in multicellular organisms. In particular, it is hypothesized that 1) the beneficial trait that was selected for in one-celled organisms was the coordinated response of ion channels to compounds that were present in the environment, which influenced the conformational equilibrium of ion channels; 2) this coordinated response prevented the deleterious consequences of entry of positive charges into the cell, thereby increasing the fitness of the organism; and 3) these compounds (which may have included organic anions, cations, and zwitterions as well as uncharged compounds) mimicked inhaled anesthetics in that they were interfacially active, and modulated ion channel function by altering bilayer properties coupled to channel function. The proposed hypothesis is consistent with known properties of inhaled anesthetics. In addition, it leads to testable experimental predictions of nonvolatile compounds having anesthetic-like modulatory effects on ion channels and in animals, including endogenous compounds that may modulate ion channel function in health and disease. The latter included metabolites that are increased in some types of end-stage organ failure, and genetic metabolic diseases. Several of these predictions have been tested and proved to be correct.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.

Evidence for two pools of cholesterol in the Acholeplasma laidlawii strain B membrane: a deuterium NMR and DSC study

Recent investigations have indicated that there exists a well-defined range of membrane hydrocarbon order compatible with good growth of the microorganism Acholeplasma laidlawii B [Monck, M., Bloom, M., Lafleur, M., Lewis, R. N. A. H., McElhaney, R. N., & Cullis, P. R. (1992) Biochemistry 31, 10037-10043]. Since cholesterol increases hydrocarbon order in membranes, it was of interest to examine the effect of cholesterol on the hydrocarbon order and growth characteristics of A. laidlawii B. Cholesterol is normally absent from A. laidlawii membranes since it is neither biosynthesized nor required for the growth or survival of the microorganism. However, cholesterol will be incorporated into the membrane if exogenously supplied to the A. laidlawii culture. For membranes prepared from cells grown in the presence of cholesterol, chemical determinations indicated cholesterol represented as much as 40 mol% of the total membrane lipid. However, 2H NMR order parameter measurements and DSC studies of the same membrane preparation suggested that cholesterol was present at significantly lower levels (approximately 10-15 mol%) in the membrane lipid bilayer. Further incorporation of cholesterol into the A. laidlawii lipid bilayer was found to occur with an increase in temperature or by lyophilization and rehydration at high temperatures, suggesting that sterol present in a separate pool in the membrane preparation could then gain access to the bilayer. 2H NMR spectra of A. laidlawii membrane preparations containing deuterium-labeled cholesterol indicate that the bulk of the cholesterol present in this separate pool is in a solid form.